1. Field of the Invention
The present invention relates to a stack sheet supplying apparatus for supplying stacked sheet one by one from an uppermost one and an image reading apparatus having such a sheet supplying apparatus.
2. Related Background Art
In the past, a sheet supplying apparatus for supplying a sheet such as an original has been used with an image forming apparatus such as a copying machine. Such a sheet supplying apparatus comprises a sheet tray on which a plurality of sheets are stacked as a sheet stack, and a sheet supply roller for supplying a sheet in the sheet stack from an uppermost one toward an image forming portion. A separation means disposed at a downstream side of the sheet supply roller in a sheet conveying direction serves to separate the sheets (when a plurality of sheets are supplied by the sheet supply roller) one by one and convey the separated sheet toward a downstream side. Further, a convey means disposed at a downstream side of the separation means serves to further convey the sheet toward the downstream side.
In the above-mentioned sheet supply roller, it is necessary to supply the sheet by a proper supplying force. To this end, various methods have been proposed.
As a first method, the sheet tray includes a lift mechanism and a height detection means for detecting a height of the sheet stack (height of an uppermost sheet) rested on the tray is provided so that, when the height of the sheet stack is decreased by supplying the sheets successively, the lift mechanism is operated in response to a signal from the height detection means to maintain the uppermost sheet in the sheet stack to the optimum height.
As a second method, a height detection means for detecting a height of the sheet stack (height of an uppermost sheet) rested on the tray is provided so that, when the height of the sheet stack is decreased by supplying the sheets successively, a sheet supply roller is brought to the optimum height in response to a signal from the height detection means.
The height detection means may be a distance measuring sensor, or a sensor of type in which the fact that a sensor flag lever is contacted with the sheet. However, in the above-mentioned first method, since the lift mechanism and the height detection means are required, the entire apparatus becomes expensive. In the above-mentioned second method (using the sensor flag lever), if the sheet is curled, the sensor will detect a curled portion of the sheet, with the result that the sheet supply roller is rotated idly without contacting with the major portion of the sheet, thereby causing poor sheet supply, or skew-feed of the sheet due to insufficient sheet supplying force of the sheet supply roller.
When the sheet is supplied, the sheet supply roller is lowered until it is contacted with the sheet stack. In this case, when the sheet supply roller is contacted with the sheet stack, vibration is normally generated due to the reaction. In such a case, if the sheet supply roller is rotated while the vibration is being generated, the sheet supply becomes unstable. Thus, the sheet supply roller is stopped until the vibration disappears.
However, when the sheet supply roller is stopped in this way, the sheet supplying time (sheet treating time) is increased. This causes a serious problem particularly when a large number of sheets are supplied.